HORANA, Sri Lanka, April 19 (Reuters) - At least five people were killed in Sri Lanka on Thursday when a worker at a rubber factory fell into a tank of ammonia and a colleague and local residents died trying to save him, police said.

Residents told media the rescuers were overcome by fumes as they tried to help the worker and they too fell into the tank, which contained liquid ammonia.

Hospital officials said 18 people had been admitted, of whom five had died.

“It may be due to some toxic gases,” Thamara Kalubowila, the director at Horana hospital, told local TV channel Derana. “So far five have died and 13 people are admitted for treatment and one is under intensive care.”

The incident happened in the town of Horana, 50 km (30 miles) from the capital, Colombo.

Television showed rescue officers wearing breathing apparatus taking the injured out of the factory and searching the ammonia tank for any other casualties.

The rescuers who died were three local residents and another factory worker.

Police spokesman Ruwan Gunasekara said the factory manager was arrested on suspicion of negligence.

Police were deployed near the factory after people living nearby protested against the suspected unsafe handling of chemicals, local media reported. Some residents demanded the closure of the factory. (Additional reporting by Ranga Sirilal Writing by Shihar Aneez Editing by Robert Birsel and Alison Williams)